Auto merge of #123778 - jhorstmann:optimize-upper-lower-auto-vectorization, r=the8472

Improve autovectorization of to_lowercase / to_uppercase functions

Refactor the code in the `convert_while_ascii` helper function to make it more suitable for auto-vectorization and also process the full ascii prefix of the string. The generic case conversion logic will only be invoked starting from the first non-ascii character.

The runtime on a microbenchmark with a small ascii-only input decreases from ~55ns to ~18ns per iteration. The new implementation also reduces the amount of unsafe code and encapsulates all unsafe inside the helper function.

Fixes #123712

Author: bors